Loading arrangement for floating production storage and offloading vessel

ABSTRACT

A loading arrangement includes a riser that extends from a subsea structure to a coupling element for coupling the riser to a vessel. The coupling element includes a buoy body which is connected to a retention member via a flexible connection part. The retention member, such as a submerged buoy, is attached to anchor lines which at or near their end parts are provided with buoy. The connection part, which can be a cable or a frame structure has a relatively high tensile strength to anchor the vessel to the sea bed and to prevent drift of the vessel when tension is exerted on the connection part and the anchor line.

BACKGROUND OF THE INVENTION

The invention relates to a loading arrangement comprising a riserextending from a subsea structure to a coupling element that is attachedto the riser for coupling the riser to a vessel, the coupling elementcomprising a buoy body that is connected to the sea bed via anchor linesthat are provided with buoyancy means at or near their ends that arelocated near the buoy body.

In the Heidrun fields, oil is transferred from the subsea well toshuttle tankers via a Direct Shuttle Loading (DSL) system. In this wayintermediate storage facilities need not be used and continuous oilproduction and transfer directly to the shuttle tanker is possible. Theshuttle tankers comprise a submerged tapered loading and mooringconstruction having a keel cavity in which a coupling buoy is received.The tapered coupling buoy is attached to flexible risers connected tothe subsea oil well and is attached to the sea bed via anchor lines. Theanchor lines are near their upper ends provided with buoyancy such thatthe coupling buoy is maintained at a predetermined position below waterlevel upon detaching from the shuttle tanker. Such a system is furtherdescribed in WO 96/36529.

During high seas, the shuttle tanker will be disconnected from thecoupling buoy, for instance at wave heights of 10 m or higher. When thewave height decreases, the shuttle tanker needs to be reattached to buoyat significant wave heights of 4-5,5 m or at higher sea states, which isa very difficult and precise operation. The horizontal and verticalposition of the detached buoy, which is suspended between the buoyantupper ends of the anchor lines, is very stable and can not follow therelative movements of the vessel during the hook-up of the tapered buoy.It is therefore an object of the present invention to provide a taperedbuoy loading arrangement which can be easily coupled to a shuttle tankerafter detachment.

SUMMARY OF THE INVENTION

Thereto the loading arrangement according to the present invention ischaracterised in that the buoy body is connected to a retention membervia a flexible connection part, the retention member being attached tothe anchor lines, wherein the connection part has a relatively hightensile strength to anchor the vessel to the seabed and to prevent driftof the vessel when tension is exerted on the connection part and theanchor lines. By the substantially flexible connection part, the taperedbuoy is decoupled from the relatively large horizontal and verticalstiffness of the anchoring means. The buoyancy means may be formed bythe retention member, which may have positive buoyancy or by separatebuoyancy members attached to the end of each anchor line, or by acombination thereof. It is not necessary for the buoy body to have a lotof buoyancy. Because of the substantial flexible connection of thetapered buoy to the retention member, the buoy is able to follow thevertical and horizontal movements of the vessel, which makes it easy topull the tapered buoy in towards the shuttle tanker and to align thebuoy with the keel cavity during the hook-up procedure. By providing asubstantially flexible connection part, the dynamic vessel is in aflexible way connected to the relatively stiff and stable mooring andloading system formed by the retention member and the anchor lines. Withthe term “flexible” it is meant a connection which can be displaced in alateral direction with respect to the vertical such as a chain or cableconnection, a pivoting frame or a tubular member which comprisespivoting segments, and the like.

It should be noted that an offshore tanker loading system in which aflexible attachment between a coupling member which is located at thewater surface for coupling to a shuttle tanker, and a submergedretention member in the form of a buoy is known from U.S. Pat. No.5,275,510. In the known loading system however the retention member isconnected to the seabed via a riser system. From the retention member asingle riser extends vertically upwards to the coupling member forproviding a fluid connection with a shuttle tanker. This system can onlybe used in combination with a dynamical positioning system in which thetanker position is maintained constant by control of the thrusters. Noanchoring forces can be transmitted through the vertical riser parttowards the seabed such that an anchoring function is not present inthis case.

In one embodiment of the present invention, the retention membercomprises a chain table connected to the seabed via at least two anchorlines. The chain table may comprise buoyancy to keep it at its desireddepth. The anchor lines can near their upper ends be provided withbuoyancy members and can extend in a circular pattern around the chaintable such that it is maintained at a predetermined depth below sealevel, for instance 50 meters at the total water depth of for instance1400 meters. The chain table may comprise a rotatable swivel having astationary part connected to the riser and a rotating part connected toa flexible riser section which extends from the rotating part to thetapered buoy. The flexible riser section is attached to the tapered buoyvia a second swivel for allowing displacement of the flexible risersection in a plane through the connection part. In this way relativerotations of the vessel with respect to the chain table can beaccommodated without exerting too large tensions on the flexible risersection between the chain table and the tapered buoy.

In another embodiment the retention member may comprise a pivot arm thatis pivotably connected to a vessel, such as a floating productionstorage and offloading vessel (FPSO) wherein the connection part isattached at or near the free end of the pivot arm. The pivot arm maycomprise a cryogenic transfer boom having two interconnected pivotingpipes. In this system the tapered buoy is permanently connected to thepivot arm and can be easily picked up in the keel cavity of the shuttletanker for offloading without the buoy being moored to the seabed. Thisembodiment is particularly useful in harsh environments and duringhigher sea states of wave heights between 6-8 m, and improves theshuttle tanker connect/disconnect sea state and thus the overallavailability of the shuttle tanker. The distance between the first andsecond vessels could be as large as 500 meters. A ballast weight may beattached to the pivot arm, which in another embodiment may for instancebe a delta frame, to stabilize the frame when the tapered buoy isdisconnected from the shuttle tanker. A further advantage of attachingthe tapered buoy to the pivot arm is that upon connection, the pivot armkeeps the shuttle tanker at a relatively fixed distance from the firstvessel (FPSO).

Preferably a weight is attached to a support arm that is locatedtransversely to the pivot arm such that the weight is located below thepivoting connection of the pivot arm when the pivot arm is in itssubmerged equilibrium position. In this way a stable submerged positionis achieved wherein the moment on the pivot hinges is relatively low.

The connection part may comprise a chain which can be provided with achain swivel for allowing rotation of an upper and lower chain part uponweathervaning of the vessel. It is also possible to use a substantiallyrigid frame member as a connection part, the frame member beingconnected to the buoy body via a pivot connection such that the buoybody may be tilted with respect to the frame member upon drift of theshuttle tanker.

In another embodiment the tapered buoy is connected to a first vesselvia a flow line which is taken up by a winch on the vessel. The buoycould be moved into the direction of the keel cavity of the shuttletanker via a hook up line, a remote operated vehicle (ROV) or withthrusters connected to the tapered buoy.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of a loading system according to the present inventionwill by way of example be explained in detail with reference to theaccompanying drawings. In the drawings:

FIG. 1 shows a side view of a loading arrangement according to thepresent invention,

FIG. 2 shows a first embodiment of the loading arrangement wherein theconnection part is formed by a cable or chain,

FIG. 3 shows an embodiment wherein the connection part is flexible andis formed by a substantially rigid frame member connected to the taperedbuoy and to a chain table via pivot connections,

FIG. 4 shows a partially cut away enlarged detail of the loadingarrangement of the present invention comprising a chain type connectionpart,

FIG. 5 shows a further embodiment of a loading arrangement of thepresent invention wherein the tapered buoy is connected to a pivot arm,

FIG. 6 shows a top view of the pivot arm of FIG. 5.

FIG. 7 shows a side view of another embodiment of a pivot arm in theform of a transfer boom, and

FIG. 8 shows a third embodiment wherein the tapered buoy is attached toa winch on a vessel via a flow line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a loading and mooring arrangement 1 according to thepresent invention wherein a chain table 2 is connected to a number ofanchor lines 3, 3′. The anchor lines 3, 3′ are connected to the seabedvia anchors such as piled anchors, suction anchors or fluke anchors. Atthe upper ends of the anchor lines 3,3′, which can comprise anchorchains, wire rope cables or cables of synthetic materials such aspolyethylene or any combination thereof, buoyancy members 4, 4′ areconnected. The anchor lines 3, 3′ extend in a circular or groupedconfiguration around the chain table 2 and maintain the chain table at apredetermined position below water level 5. A tapered buoy body, orsubmerged turret loading buoy (STL) 7 is attached to the chain table 2via a flexible connection part 8 which can be in the form of a chain,cable, or pivoting frame member. The tapered buoy 7 comprises couplingmembers for attaching to a keel cavity 6 in the shuttle tanker 9. Thekeel cavity 6 may be part of a turret system around which the tanker 9can weathervane or can be fixedly placed in the hull of the vesselwithout the use of a turret construction.

A number of risers 10, of which only one has been shown for reasons ofclarity, extend from the seabed, for instance from a subsea oil well tothe chain table 2. From the chain table 2 a flexible riser section 11extends towards the tapered buoy 7. The length of the connection part 8may for instance be between 10 and 50 meter. The water depth in whichthe system is used may for instance be 1300 meter.

FIG. 2 shows an embodiment wherein the connection part 8 is formed by achain or cable such that a large degree of freedom in positioning thetapered-buoy 7 with respect to the relatively stiffly supported chaintable 2 is possible.

In the embodiment of FIG. 3, the connection part 8 is formed by asubstantially rigid tubular member or frame member 14 which is attachedto the tapered buoy 7 via a pivot joint 15. At the bottom, the tubularmember or frame member 14 is connected to the chain table 2 via a pivotjoint 16. However, the pivot joint 16 is optional and may be omitted.The riser 10 may be guided through the frame member 14 or could berouted outside of the frame member 14.

FIG. 4 shows an enlarged detail of the loading and mooring arrangementaccording to the present invention wherein the product riser 10 isconnected to a stationary part of a fluid swivel 16 on the chain table2. The rotating part of the fluid swivel 16 is connected to flexibleriser section 11. At the lower end of the tapered-buoy 7, the flexibleriser section 11 is connected to a swivel 17. The chain 19, connectingthe chain table 2 and the buoy 7, is provided with a chain swivel 20such that an upper chain section 21 can rotate relative to lower chainsection 22 around the length dimension of the chain 19. At the upperend, the chain 21 is connected to a gimbal table 23 of the tapered-buoy7.

FIG. 5 shows an embodiment wherein the tapered-buoy 32 is connected viaa connection part 32′, which in this case may also be a cable, chain orpivoting frame member, to the end of a pivot arm 34. The buoy 32 can beengaged with a keel cavity 37 in the shuttle tanker 31. As can be seenin FIG. 6, the pivot arm 34 has the form of a delta-type frame. The arm34 is connected to a floating production storage and offloading vessel30 via a pivot connection 30′. Transverse arms 34′, 34″, which extendtransversely to the delta frame 34, are connected to a positioningweight 33 which in the rest position is located directly below the pivotconnection 30′. A flow line 35 extends along the arm 34 to thetapered-buoy 32.

FIG. 7 shows an embodiment wherein the pivot arm 34 is formed by acryogenic LNG-boom having two arm sections 34′, 34″ which are connectedin a pivot joint 39′. At the side of the vessel 30 the verticallyextending arm section 34′ is connected to a swivel 39. At the end of thesecond arm section 34″, the buoy 32 is connected to a flexible memberwhich is comprised of swivels 38, 38′, 38″ allowing rotational movementaround an axis along the length direction of arm section 34″, around anaxis perpendicular to the plane of the drawing and around an axisparallel to the centre line of the buoy 32, respectively. A cryogenicLNG-boom of this type is described in detail in International patentapplication number PCT/EP99/01405 in the name of the applicant.

Finally, FIG. 8 shows an embodiment wherein a tapered buoy 40 isconnected to a flow line 41, which has no positive buoyancy. Flow line41 is collected on a winch 42 on the FPSO-vessel 43. A shuttle tanker 44having a dynamic positioning system in the form of multiple thrusters 45can be manoeuvred in the proximity of the FPSO-vessel 43 and can attachto the buoy 40 via a hook-up line 47. With the hook-up line 47 thetapered buoy 40 can be winched into the keel cavity 48 for connectingthe flow line 41 to the shuttle tanker 44. No anchoring function of theflow line 41 and the buoy 40 are provided in this case, the dynamicpositioning system of the shuttle tanker 44 maintaining the properrelative position of the tanker 44 with respect to the FPSO-vessel 43.

What is claimed is:
 1. A loading arrangement (5) comprising: a firstvessel (31) having a cavity (37) near keel level that can be coupled toa coupling element comprising a buoy body (32) and a flexible connectionpart (32′) having a relatively high tensile strength to anchor the firstvessel (31) to a mooring device via the buoy body (32), the mooringdevice comprising, a second vessel (30) having a pivot arm (34) that ispivotably connected to the second vessel (30), the flexible connectionpart (32′) being connected at or near a free end of the pivot arm (34),and a fluid path (35) extending from the second vessel (30), via thepivot arm (34) to the buoy body (32).
 2. The loading arrangement (36)according to claim 1, wherein the pivot arm (34) comprises a weight (33)for maintaining the pivot arm in a stabilized submerged position.
 3. Theloading arrangement (36) according to claim 2, wherein the weight (33)is carried by a support arm (34′, 34″) that is attached transversely tothe pivot arm (34) such that the weight (33) is located below a pivotingconnection (30′) of the pivot arm (34) to the second vessel (30) whenthe arm (34) is in its submerged equilibrium position.
 4. The loadingarrangement of claim 1, further comprising a weight attached to saidpivot arm that urges said free end underwater.
 5. A loading arrangement,comprising: a moored vessel from which a fluid is to be moved; a pivotarm pivotally attached to said moored vessel, said pivot arm having amovable free end; a buoy body attached to said movable free end by aflexible connection member that holds said buoy body underwater, saidbuoy body being arranged and adapted to be received in a cavity at ornear keel level in a further vessel to which the fluid is to be moved;and a path for the fluid extending from said moored vessel through saidpivot arm to said buoy body.
 6. The loading arrangement of claim 5,further comprising a weight attached to said pivot arm that urges saidfree end underwater.